Process for the preparation of foundry cores and molds

ABSTRACT

This invention relates to a process for the preparation of foundry cores and molds by mixing a filler and a casting resin mixture and then curing the resultant mixture. The casting resin mixture is made from (a) 25 to 99.9% of at least one organic polyisocyanate, (b) 0.1 to 50% of at least one organic compound containing at least two epoxide groups, and (c) up to 50% of a polyhydroxyl compound, the quantities being based on the total quantity of resin.

This application is a continuation of application Ser. No. 07/581,484filed Sep. 12, 1990.

BACKGROUND OF THE INVENTION

This invention relates to a process for the preparation of foundry coresand molds.

Curable resins are widely used in the foundry industry for making moldsand cores which predetermine or produce the dimensions and cavities ofthe castings. Of particular importance in this regard is hot-curing andcold-curing mold and core sand binders based on phenolic, furan, andamino resins which contribute considerably towards economicmass-production casting and towards improving quality. At present,energy costs are closely related to process costs. Accordingly,processes in which as much energy as possible is saved are graduallyacquiring a priority position. To help satisfy the need to save energyand costs, even with large series, so-called cold-box processes have inrecent years been increasingly significant for the production of cores.Based on the use of curable resins, these processes represent agenerally new processing variant in which one of the components isintroduced into the curing reaction as a gas or aerosol. In foundryparlance, the polyurethane blowing process is known as a cold-boxprocess and has been introduced under this name into the German foundryindustry. According to the process described in German Auslegeschriften1,483,521 and 2,011,365, polyhydroxyl compounds and polyisocyanates aremixed with the core sand and then crosslinked with an amine-basedblowing agent as catalyst, thereby forming a polyurethane. The preferredpolyhydroxyl compounds used in this process are resols produced, forexample, using cobalt or lead naphthenate as catalyst and are oftenreferred to in the relevant literature as benzyl ether resins. Thesecompounds must be free from water because water would react prematurelywith the polyisocyanate. To adjust viscosity to 130 to 450 mPa·s, thephenol resols contain approximately 30 to 35% by weight high-boilingsolvents, such as aromatic and aliphatic hydrocarbons, esters, ketones,and the like. The polyisocyanates used in this process are generallydiphenylmethane-4,4'-diisocyanate or derivatives thereof which are alsodiluted with solvents of the above-mentioned type. The cold-box bindersare generally formulated in such a way that they are mainly used in aratio of 1:1 with polyisocyanate. The total quantity of binder(including solvent) is about 1 to 2% by weight, whereas the totalquantity of catalyst is about 0.02 to 0.10% by weight (based in bothcases on the amount of sand). The process is carried out by initiallyshooting the core sand into the core box and then blowing in a mixtureof amine and air as a gas or aerosol. The amines used in the process aregenerally triethylamine, dimethylethylamine, or dimethylisopropylaminewhich are introduced into the core box under a pressure of 0.2 to 2 bar.The residual gases are removed from the core with heated purging air orcarbon dioxide gas and may be treated in an acid scrubber charged withdilute sulfuric acid or phosphoric acid. Suitable scrubbers work on thecountercurrent principle. Significant advantages of this process includea considerable increase in productivity, smooth core surfaces by virtueof the excellent flowability of the sand during shooting, highdimensional accuracy through cold curing, and high strengths of the moldmaterials despite extremely short curing times. A disadvantage of thisprocess is the limited storage life of the core sand because, even inthe absence of amine, certain polyurethane polyaddition reactions beginin the core sand. In addition, the solvents of the binder andpolyisocyanate solutions partly evaporate at the surface. Consequently,a reduction in strength occurs if the sand is processed a few hoursafter preparation. Thus, the cold flexural strength measuresapproximately 5.5 N/mm when blowing is carried out immediately afterpreparation of the sand but only 4 N/mm when blowing is carried out onehour after preparation of the sand. Reductions in strength of 25% and60% are observed after 2 hours and 3 hours, respectively. In addition,the sand is impossible to process about 8 hours after preparationbecause it has solidified as the polyurethane reaction proceeds.

Accordingly, the problem addressed by the present invention was todevelop a casting resin mixture which gives improved storage life forthe prepared core sand without a reduction in the strength after a fewhours. An extended processing time such as provided by the presentinvention affords considerable advantage in the production process usedfor the core sands and leads to a significant reduction in productioncosts because the core sand needs to be prepared with the polyurethaneraw materials only once a day or less. Another problem addressed by thepresent invention was to develop core sand binders having higherstrength values and higher heat resistance levels, which are needed inparticular for more recently developed foundry applications.

SUMMARY OF THE INVENTION

The present invention relates to a process for the preparation offoundry cores and molds comprising

(1) combining a filler and a casting resin mixture, wherein said castingresin mixture comprises

(a) 25 to 99.9%, based on the total quantity of resin, of at least oneorganic polyisocyanate,

(b) 0.1 to 50%, based on the total quantity of resin, of at least oneorganic compound containing at least two epoxide groups,

(c) 0 to 50%, based on the total quantity of resin, of a polyhydroxylcompound, and

(d) optionally, auxiliaries, additives, and solvents; and

(2) curing the combined filler and casting resin mixture.

In one preferred embodiment, the ratio of reactive isocyanate groups tohydroxyl groups in the casting resin mixture is greater than about 1.1:1(more preferably, greater than 1.5:1).

A particularly suitable filler is sand, although silica powder, chalk,aluminum oxide, corundum, metal powders, ceramic powders, or evenplastic powders are also suitable.

DETAILED DESCRIPTION OF THE INVENTION

It may be regarded as particularly surprising that the additional use,relative to the prior art, of an organic compound containing at leasttwo epoxide groups should lead to outstanding storage stabilities andstrength values. It may also be regarded as particularly surprisingthat, despite a ratio of reactive isocyanate to hydroxyl groups in thecasting resin mixture of at least 1.1:1 (preferably at least 1.5:1),improved storage stability and increased strength values are alsoobserved precisely where, according to the prior art, lower strengthvalues are obtained through over-crosslinking or under-crosslinking ofpolyurethane resins. See Gardziella in Becker/Braun, Kunststoffhandbuch,Vol. 10, Duroplaste, page 976. Polyhydroxyl compounds need not bepresent at all because curing is readily attained solely by aminecatalysis of the isocyanate epoxide mixture for processing times ofseveral hours. It is also surprising that the blowing time (i.e., thecatalysis time) of these mixtures need not extended in comparison withthe prior art, as might have been expected from a knowledge ofmacromolecular chemistry.

In one preferred embodiment, at least part of component (a) is mixedwith at least part of component (b) in a preliminary reaction step andthe resulting mixture is made stable in storage by the use of a reactioninhibitor. In particular, at least part of component (a) can be mixedwith at least part of component (b) in a preliminary reaction step toform an intermediate product containing oxazolidinone and isocyanurategroups. This preliminary reaction is stopped when no more than 65% ofthe isocyanate groups present in the starting mixture are converted tothe intermediate product by addition of a reaction inhibitor so that thereaction mixture remains stable in storage. Despite the use of reactioninhibitors, re-catalysis by blowing with the amine is readily possible.

Starting component (a) may be selected from any organic polyisocyanatesof the type known from polyurethane chemistry. Suitable organicpolyisocyanates include aliphatic, cycloaliphatic, araliphatic,aromatic, and heterocyclic polyisocyanates of the type described, forexample, by W. Siefken in Justus Liebigs Annalen der Chemie, 562,75-136. Suitable polyisocyanates include those corresponding to theformula

    Q(NCO).sub.n

wherein n is a number from 2 to 4 (preferably 2), and Q represents analiphatic hydrocarbon group containing from 2 to about 18 (preferablyfrom 6 to 10) carbon atoms, a cycloaliphatic hydrocarbon groupcontaining from 4 to about 15 (preferably from 5 to 10) carbon atoms, anaromatic hydrocarbon group containing from 6 to about 15 (preferably 6to 13) carbon atoms, or an araliphatic hydrocarbon group containing from8 to about 15 (preferably from 8 to 13) carbon atoms. Examples ofsuitable polyisocyanates include ethylene diisocyanate,1,4-tetramethylenediisocyanate, 1,6-hexamethylenediisocyanate,1,12-dodecanediisocyanate, cyclobutane-1,3-diisocyanate,cyclohexane-1,3- and -1,4-diisocyanate and mixtures thereof,1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (GermanAuslegeschrift 1,202,785 and U.S. Pat. No. 3,401,190), 2,4- and2,6-hexahydrotoluene diisocyanate and mixtures thereof, hexahydro-1,3-and/or 1,4-phenylene diisocyanate, 1,3- and 1,4-phenylenediisocyanate,2,4- and 2,6-toluenediisocyanate and mixtures of these isomers,diphenylmethane-2,4'- and/or -4,4'-diisocyanate and otherpolyisocyanates of the diphenylmethane series, andnaphthylene-1,5-diisocyanate.

It is also possible to use, for example,triphenylmethane-4,4',4"-triisocyanate, polyphenyl polymethylenepolyisocyanates of the type obtained by condensing aniline withformaldehyde, followed by phosgenation (British Patents 874,430 and848,671), m- and p-isocyanatophenylsulfonyl isocyanates (U.S. Pat. No.3,454,606), perchlorinated aryl polyisocyanates (U.S. Pat. No.3,277,138), polyisocyanates containing carbodiimide groups (U.S. Pat.No. 3,152,162), norbornane diisocyanates (U.S. Pat. No. 3,492,330),polyisocyanates containing allophanate groups (British Patent 994,890),polyisocyanates containing isocyanurate groups (U.S. Pat. No.3,001,973), polyisocyanates containing acylated urea groups (GermanPatentschrift 1,230,778), polyisocyanates containing bioret groups (U.S.Pat. Nos. 3,124,605, 3,201,372, and 3,124,605), polyisocyanates producedby telomerization reactions (U.S. Pat. No. 3,654,106), polyisocyanatescontaining ester groups (U.S. Pat. No. 3,567,763), reaction products ofthe above-mentioned isocyanates with acetals (German Patentschrift1,072,385), and polyisocyanates containing polymeric fatty acid esters(U.S. Pat. No. 3,455,883).

It is also possible to use distillation residues containing isocyanategroups which are obtained in the production of isocyanates on anindustrial scale, optionally in solution in one or more of theabove-mentioned polyisocyanates. Mixtures of the polyisocyanatesmentioned above may also be used.

In general, it is particularly preferred to use the commercially readilyavailable polyisocyanates, for example, 2,4- and 2,6-toluenediisocyanate and any mixtures of these isomers ("TDI"),diphenylmethane-2,4'- and/or -4,4'-diisocyanate ("MDI"), polyphenylpolymethylene polyisocyanates of the type obtained by phosgenation ofaniline-formaldehyde condensates ("crude MDI"), and polyisocyanatescontaining carbodiimide groups, urethane groups, allophanate groups,isocyanurate groups, urea groups, or bioret groups ("modifiedpolyisocyanates"), particularly modified polyisocyanates of the typederived from 2,4- and/or 2,6-toluene diisocyanate or from 4,4'- and/or-2,4'-diphenylmethane diisocyanate.

Component (b) may be selected from any aliphatic, cycloaliphatic,aromatic, or heterocyclic compound containing at least two epoxidegroups, preferably 1,2-epoxide groups. Preferred polyepoxides suitableas component (b) contain 2 to 4 (preferably 2) epoxide groups permolecule and have an epoxide equivalent weight of about 90 to about 500(preferably 170 to 220).

Suitable polyepoxides (b) include polyglycidyl ethers of polyhydricphenols, such as pyrocatechol, resorcinol, hydroquinone,4,4'-dihydroxydiphenylmethane,4,4'-dihydroxy-3,3'-dimethyldiphenylmethane, of4,4'-dihydroxydiphenylmethane, 4,4'-dihydroxydiphenylcyclohexane, of4,4'-dihydroxy-3,3'-dimethyldiphenylpropane, 4,4'-dihydroxybiphenyl,4,4'-dihydroxydiphenyl sulfone, and tris(4-hydroxyphenyl)methane, aswell as chlorination and bromination products of the above-mentioneddiphenols; of novolaks (i.e., reaction products of monohydric orpolyhydric phenols with aldehydes, preferably formaldehyde, in thepresence of acidic catalysts); of diphenols obtained by esterificationof 2 moles of the sodium salt of an aromatic hydroxycarboxylic acid with1 mole of a dihaloalkane or dihalodialkyl ether (see British Patent1,017,612); or of polyphenols obtained by condensation of phenols andlong-chain haloparaffins containing at least two halogen atoms (seeBritish Patent 1,024,288). Other suitable polyepoxides includepolyepoxide compounds based on aromatic amines and epichlorohydrin, suchas N-di(2,3-epoxypropyl)aniline,N,N'-dimethyl-N,N'-diepoxypropyl-4,4'-diaminodiphenylmethane,N-(diepoxypropyl)-4-aminophenyl glycidyl ether (see British Patents772,830 and 816,923).

Also suitable for use as component (b) are glycidyl esters of polybasicaromatic, aliphatic, and cycloaliphatic carboxylic acids. Examples ofsuch glycidyl esters include phthalic acid diglycidyl ester, adipic aciddiglycidyl ester, glycidyl esters of reaction products of 1 mole of anaromatic or cycloaliphatic dicarboxylic anhydride and 1/2 mole of a diolor 1/n mole of a polyol containing n hydroxy groups, andhexahydrophthalic acid diglycidyl esters optionally substituted bymethyl groups.

Glycidyl ethers of polyhydric alcohols, such as butane-1,4-diol,butene-1,4-diol, glycerol, trimethylolpropane, pentaerythritol, andpolyethylene glycols, may also be used. Triglycidyl isocyanurate,N,N'-diepoxypropyl oxamide, polyglycidyl thioethers of polyfunctionalthiols such as bis(mercaptomethyl)benzene, diglycidyl trimethylenetrisulfone, and polyglycidyl ethers based on hydantoins, are alsosuitable.

It is also possible to use epoxidation products of polyunsaturatedcompounds, such as vegetable oils and their conversion products;epoxidation products of diolefins and polyolefins, such as butadiene,vinyl cyclohexene, 1,5-cyclooctadiene, and 1,5,9-cyclododecatriene;epoxidation products of polymers and copolymers containing epoxidizabledouble bonds, such as those based on polybutadiene, polyisoprene,butadienestyrene copolymers, divinylbenzene, dicyclopentadiene, andunsaturated polyesters; epoxidation products of olefins such as thoseobtained by Diels-Adler addition and subsequent conversion intopolyepoxides by epoxidation with per compounds; and epoxidation productsof compounds which contain two cyclopentene rings or cyclohexene ringsattached by bridge atoms or groups or bridge atoms. Polymers ofunsaturated monoepoxides, for example, of methacrylic acid glycidylester or allyl glycidyl ether, are also suitable.

Preferred polyepoxide compounds or mixtures used as component (b)include polyglycidyl ethers of polyhydric phenols, particularlybisphenol A; polyepoxide compounds based on aromatic amines, moreparticularly bis(N-epoxypropyl)aniline,N,N'-dimethyl-N,N'-diepoxypropyl-4,4'-diaminodiphenylmethane, andN-diepoxypropyl-4-aminophenyl glycidyl ether; polyglycidyl ethers ofcycloaliphatic dicarboxylic acids, more particularly hexahydrophthalicacid diglycidyl esterl; and polyepoxides of the reaction product of nmoles of hexahydrophthalic anhydride and I mole of a polyol containing nhydroxyl groups (wherein n is an integer of 2 to 6), more particularly 3moles of hexahydrophthalic anhydride and 1 mole of1,1,1-trimethylolpropane,3,4-epoxycyclohexylmethane-3,4-epoxycyclohexanecarboxylate.

In special cases, liquid polyepoxides or low viscosity diepoxides, suchas bis(N-epoxypropyl)aniline or vinyl cyclohexane diepoxide, can furtherreduce the viscosity of polyepoxides that are already liquid or canconvert solid polyepoxides into liquid mixtures.

One preferred embodiment is characterized by the use of a casting resinmixture consisting of component (a), component (b), and, optionally,component (d). In this embodiment, about 50 to about 99% by weight ofcomponent (a) and about 0.1 to about 50% by weight of component (b)(based on the casting resin mixture) are preferably used.

Suitable polyhydroxyl compounds (c) according to the invention includelow- to medium-viscosity linear or branched polyether polyols and/orpolyester polyols containing primary and/or secondary hydroxyl groups.Polyether polyols can be obtained in known manner by reaction ofpolyfunctional starter molecules, such as ethylene glycol, propyleneglycol, glycerol, 1,4-butanediol, trimethylolpropane, pentaerythritol,sorbitol, hexanetriol, and the like, or mixtures thereof, with ethyleneoxide and/or propylene oxide. Polyester polyols are formed in knownmanner by reaction of polyalcohols of the large methyl- and polylyetherpolyol types described above (or mixtures thereof) with organicsaturated and/or unsaturated polycarboxylic acids, including adipicacid, sebacic acid, phthalic acid, tetrahydrophthalic acid,hexahydrophthalic acid, maleic acid, and fumaric acid, or mixturesthereof. Polymers containing hydroxyl groups, preferably anhydrousphenolic resins, may also be used as the polyhydroxyl compounds. Specialexamples of such resins are benzyl ether phenolic resins, novolaks,resols, or substituted phenolic resins. The benzyl ether phenolic resinsmay be obtained by reaction of an optionally substituted phenol with analdehyde, preferably formaldehyde, using a molar ratio of aldehyde tophenol greater than 1:1. The reaction is carried out in substantiallyanhydrous liquid phase at temperatures below about 130° C. in thepresence of catalytic quantities of soluble metal salts dissolved in thereaction medium. Suitable solvents include naphthenates, neodecanoates,octoates, or lactates of lead, calcium, zinc, tin, manganese, copper, ormagnesium. Cresol resins that are also suitable as polyhydroxylcompounds may be produced in the same way as the benzyl ether phenolicresins described above, except that the reaction of the phenol with thealdehyde is carried out in aqueous phase in an alkaline medium. Thesematerials are substantially neutral and contain a considerablepercentage of phenolic hydroxyl groups. In general, these resol resinsor benzyl ether phenolic resins preferably have a ratio of phenolichydroxyl groups to aliphatic hydroxyl groups of at least 3:1.

Suitable auxiliaries and additives (d) include inorganic or organicpigments or plasticizers, such as dioctyl phthalate, tributyl phosphate,and triphenyl phosphate. Preferred solvents (d) are high-boilingsolvents, such as aromatic and aliphatic hydrocarbons, esters, ketones,and the like.

Mixtures of 1,2-epoxides and polyisocyanates, optionally together withpolyhydroxyl compounds, have previously been described, for example, inGerman Auslegeschrift 1,115,922 and U.S. Pat. No. 4,582,723. Theseresins are used as electrical insulating materials for variousapplications. German Auslegeschrift 3,600,764 (believed to correspond toU.S. Pat. No. 4,728,676) discloses hot-curing casting resin mixturescontaining an organic polyisocyanate, at least one organic compoundcontaining two epoxide groups, a heat-activated catalyst, and,optionally, other auxiliaries and additives. European Patent Application129,799 (believed to correspond to U.S. Pat. No. 4,562,227) describeshot-curing casting resin mixtures based on polyfunctional isocyanates asprepolymers in the form of reaction products of diphenylmethanediisocyanate and a diol and also a polyfunctional epoxy resin. None ofthe patents described above mentions the use of these casting resinmixtures as a binder for foundry sands. On the contrary, the patents alldisclose the use of the materials as casting or impregnating resins forthe electrical industry and recommend initial curing or full curing attemperatures above 100° C. to 200° C. or 250° C. This does notcorrespond to the cold-box process of the present invention, in whichboth the reaction and the curing process take place at room temperaturesolely under the effect of the amine catalyst (which is generallyintroduced in a stream of air).

Storable mixtures of part of component (a) and at least part ofcomponent (b), which are used in one particular embodiment of theapplication, are described in German Offenlegungsschrift 3,807,660.Mixtures of at least part of component (a) with at least part ofcomponent (b) that react in a preliminary reaction step to form anintermediate product containing oxazolidinone and isocyanurate groups(the reaction being terminated at a conversion of no more than 65% ofthe isocyanate groups present in the starting mixture by addition of areaction inhibitor) and which are used in another particular embodimentof the present invention, are the subject of German Offenlegungsschrift3,644,382 (believed to correspond to U.S. Pat. No. 4,788,224). Thesepatents, however, do not disclose the process of the present inventionfor the preparation of foundry cores and molds.

The casting resin mixture is normally used as a two-component system,particularly when polyhydroxyl compounds are used. The two componentsare combined just before processing and mixing with the sand. The firstcomponent is normally the organic polyisocyanate, whereas the secondcomponent is the polyhydroxyl compound. The organic compound containingepoxide groups may be added both to the polyhydroxyl compoundand--taking into account German Offenlegungsschriften 3,644,382 and3,807,660 discussed above--to the organic polyisocyanate. As alreadymentioned, both components are normally diluted to processableviscosities by addition of the solvents described above. Without usingpolyhydroxyl compounds, it is possible--again taking into account GermanOffenlegungsschriften 3,644,382 and 3,807,660 discussed above--toprepare a storable one-component mixture that represents a furthersimplification of the sand preparation process.

The preparation of binder-sand mixtures of types different from thepresent invention and subsequent processing to core sands are describedin detail in U.S. Pat. No. 3,409,579. In this method, quartz sand havinga grain size of H 32 or H 31 should be used for preparing binder-sandmixtures. Any blade and paddle mixer may be used for mixing. The sand isinitially introduced and the components of the casting resin mixture arethen added, followed by mixing for about 1 to 2 minutes. Thefree-flowing sand mixture is then ready for processing, for example,within the mold of a standard cold-box unit. In particular, a coreshooting machine having a suitable cold-box attachment is used for coreproduction. By virtue of the very favorable flow properties of the sand,the shooting pressure may be reduced to between about 3 and 4atmospheres gauge pressure, depending on the type of core, withoutadversely affecting the quality of the core surface. Core boxes of castepoxy resin and wood may be used. The proportion of binder, based on theamount of sand, is preferably about 0.25 to 5% by weight. The liquidcatalyst is introduced into the core using an inert carrier gas, such asair, and cures the core instantly on contact with the hardening agent.In this regard, it is important that the catalyst reach all parts of thecore, which is achievable, for example, by mandrel blowing and ventingthrough appropriate nozzles. Optimal conditions must generally beestablished for each individual case. The catalyst is sprayed with a gasstream under a pressure of about 1.5 to 2 atmospheres gauge pressure. Afew minutes and, optimally, approximately 1 minute should normally besufficient. Subsequent injection of gas at about 4 to 6 atmospheresgauge pressure is advisable to assure thorough distribution of thecatalyst and simultaneous removal of any excess through the nozzles. Theperiod for which air is subsequently blown in should always be abouttwice as long as the injection times with the catalyst. The quantity ofcatalyst depends almost solely on the type and size of the core and,accordingly, is best experimentally determined. Suitable catalysts aretertiary amines, preferably tertiary amines of low volatility, such astrimethylamine, triethylamine, dimethylethylamine,dimethylisopropylamine, dimethylethanolamine, dimethylbenzylamine, andsimilar products.

Foundry cores and molds obtained by the process of the invention can beused to produce castings by introducing a foundry mix into the foundrycore or mold.

The following examples further illustrate details for the process ofthis invention. The invention, which is set forth in the foregoingdisclosure, is not to be limited either in spirit or scope by theseexamples. Those skilled in the art will readily understand that knownvariations of the conditions of the following procedures can be used.Unless otherwise noted, all temperatures are degrees Celsius and allpercentages are percentages by weight.

EXAMPLES Examples 1 to 5 General Procedure for Core Sand Preparation andCore Production

Sand having a grain size of H 31 (1,000 g) was introduced and thoroughlystirred with 30 g of each binder mixture described in detail below. Thesand-binder mixture was introduced into the mold of a standard cold-boxunit and treated for 60 seconds with a mixture of triethylamine and air.The finished, bound sand core was then removed from the mold andevaluated for appearance and strength.

The sand was stored in sealed containers at room temperature. Aftercertain intervals (see Table below), the flow properties of the sand andthe quality of a cured sand core were evaluated.

Description of the Binders

All of the binders were used as 80% solutions in a mixture of aromaticalkyl compounds. In all examples cumene is used as solvent.

BM 1

80 parts mixture of diphenylmethane-2,4'- and -4,4'-diisocyanate ("2,4'-and 4,4'-MDI") in a ratio of 1:1

20 parts bisphenol A diglycidyl ether

The mixture was made stable in storage by addition of 1,000 ppm ofp-toluenesulfonic acid methyl ester at 100° C. See GermanOffenlegungsschrift 3,807,660.

BM 2

80 parts mixture of 2,4'- and 4,4'-MDI

20 parts bisphenol A diglycidyl ether

The mixture was reacted by adding 0.5 parts of dimethyl benzene amine at100° C. to an NCO content of 20 Z and then made stable in storage inaccordance with U.S. Pat. No. 4,788,224 (see also GermanOffenlegungsschrift 3,644,382) by addition of 100° C. ppm ofp-toluenesulfonic acid methyl ester at 100 ° C.

BM 3

60 parts mixture of 2,4'- and 4,4'-MDI

40 parts bisphenol A diglycidyl ether

The mixture was made stable in storage as for binder mixture BM 1 byaddition of p-toluenesulfonic acid methyl ester.

BM 4

100 parts binder mixture BM 3

20 parts polyether polyol of propoxylated triethylolpropane (OH value of11%)

BM 5

95 parts mixture of 2,4'- and 4,4'-MDI

5 parts bisphenol A diglycidyl ether

BM 6

99.2 parts mixture of 2,4'- and 4,4'-MDI

0.8 parts bisphenol A diglycidyl ether

                                      TABLE                                       __________________________________________________________________________    Binder Mixture:                                                                             BM 1   BM 2   BM 3   BM 4   BM 5   BM 6                         __________________________________________________________________________    Evaluation of sand-binder                                                     mixture                                                                       After 1 hr    Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                 After 8 hr    Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                 After 24 hr   Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                 After 48 hr   Free-flowing                                                                         Sticks Free-flowing                                                                         Free-flowing                                                                         Free-flowing                                                                         Free-flowing                 After 72 hr   Free-flowing                                                                         Sticks Sticks Free-flowing                                                                         Free-flowing                                                                         Free-flowing                 Evaluation of the cured sand                                                  cores (processing after                                                       indicated time)                                                               1 hr          Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                   8 hr          Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                   24 hr         Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                                                                           Acceptable                   After preparation of the                                                      sand-binder mixture                                                           Heat resistance (T.sub.G, °C.).sup.(1)                                                300    150   >300    250   >300   >300                         of the pure resin mixture                                                     E modulus of the pure resin                                                                 3500   3500   3500   3500   3500   3400                         mixture                                                                       __________________________________________________________________________     .sup.(1) T.sub.G : glass transition temperature                          

What is claimed is:
 1. In a method for the preparation of foundry coresand molds comprising combining a filler and a casting resin mixture andcuring the combined filler and casting resin mixture at room temperaturein the presence of an amine catalyst which is introduced as a gas or anaerosol, the improvement wherein said casting resin mixture comprises(a)50 to 99.2%, based on the total quantity of resin, of at least oneorganic polyisocyanate, (b) 0.8 to 40%, based on the total quantity ofresin, of at least one organic compound containing at least two epoxidegroups, and (c) 0 to 17%, based on the total quantity of resin, of apolyhydroxyl compound.
 2. A process according to claim 1 wherein thecasting resin mixture additionally comprises(d) auxiliaries, additives,and solvents.
 3. A process according to claim 1 wherein the filler issand.
 4. A process according to claim 1 wherein the combined filler andcasting resin mixture is introduced into a mold of a cold-box unitbefore curing.
 5. A process according to claim 4 wherein the combinedfiller and casting resin mixture is cured in said cold-box unit byintroduction of a catalyst.
 6. A process according to claim 5 whereinthe catalyst is introduced with a carrier gas.
 7. A process according toclaim 1 wherein the ratio of reactive isocyanate groups to hydroxylgroups in the casting resin mixture is greater than 1.1:1.
 8. A processaccording to claim 1 wherein the ratio of reactive isocyanate groups tohydroxyl groups in the casting resin mixture is greater than 1.5:1.
 9. Aprocess according to claim 1 wherein the casting resin mixture consistsof components (a) and (b).
 10. A process according to claim 2 whereinthe casting resin mixture consists of components (a), (b), and (d). 11.A process according to claim 1 wherein the organic polyisocyanate (a)corresponds to the formula

    Q(NCO).sub.n

wherein n is a number from 2 to 4 and Q is an aliphatic hydrocarbongroup containing from 2 to 18 carbon atoms, a cycloaliphatic hydrocarbongroup containing from 4 to 15 carbon atoms, an aromatic hydrocarbongroup containing from 6 to 15 carbon atoms, or an araliphatichydrocarbon group containing from 8 to 15 carbon atoms.
 12. A processaccording to claim 1 wherein the organic polyisocyanate (a) is apolyisocyanate of the diphenylmethane series.
 13. A process according toclaim 12 wherein the polyisocyanate of the diphenylmethane series is4,4'- and/or -2,4'-diphenylmethane diisocyanate.
 14. A method for theproduction of castings comprising introducing a foundry mix into afoundry core or mold prepared by the process of claim
 1. 15. A processfor the preparation of foundry cores and molds comprising combining afiller and a casting resin mixture and curing the combined filler andcasting resin mixture at room temperature in the presence of an aminecatalyst which is introduced as a gas or an aerosol, wherein saidcasting resin mixture comprises(a) 50 to 99.2%, based on the totalquantity of resin, of at least one organic polyisocyanate, (b) 0.8 to40%, based on the total quantity of resin, of at least one organiccompound containing at least two epoxide groups, and (c) 0 to 17%, basedon the total quantity of resin, of a polyhydroxyl compound;wherein atleast part of component (a) is mixed with at least part of component (b)in a preliminary reaction to form an intermediate composition andwherein a reaction inhibitor is added to terminate said preliminaryreaction and to convert said intermediate composition into a storableform.
 16. A process for the preparation of foundry cores and moldscomprising combining a filler and a casting resin mixture and curing thecombined filler and casting resin mixture at room temperature in thepresence of an amine catalyst which is introduced as a gas or anaerosol, wherein said casting resin mixture comprises(a) 50 to 99.2%,based on the total quantity of resin, of at least one organicpolyisocyanate, (b) 0.8 to 40%, based on the total quantity of resin, ofat least one organic compound containing at least two epoxide groups,and (c) 0 to 17%, based on the total quantity of resin, of apolyhydroxyl compound;wherein at least part of component (a) is mixedwith at least part of component (b) in a preliminary reaction to form anintermediate composition containing oxazolidinone and isocyanurategroups and wherein a reaction inhibitor is added to terminate saidpreliminary reaction at a conversion of no more than 65% of theisocyanate groups originally present, thereby converting saidintermediate composition into a storable form.